1. Field of the Invention
The present invention relates to a touch panel used to operate a variety of electronic apparatuses.
2. Background Art
Following the advancement in recent years of various electronic apparatuses such as mobile phones, car navigation systems and the like toward high performance and diversification, there is a continuous growth in number of the electronic apparatuses equipped with optically transparent touch panels in front of their display devices such as liquid crystal panels. A user of any such electronic apparatus operates it by pressing a touch panel with a finger, a pen, or the like tool while visually observing through the touch panel a screen on the display device provided behind the touch panel. Operation of the electronic apparatus is switched in this manner from one function to another. A demand thus exists for such touch panels that are superior in visibility and reliable in operation.
With reference to FIG. 8 and FIG. 9, description is provided of a conventional touch panel.
FIG. 8 is a plan view of a conventional touch panel, and FIG. 9 is a sectional view of a main portion of the conventional touch panel. In FIG. 8 and FIG. 9, there are film-like upper substrate 101 having optical transparency, and lower substrate 102 also having optical transparency placed under upper substrate 101. Upper substrate 101 is provided with upper conductive layer 103 having optical transparency made from a material such as indium tin oxide, and formed on a back surface thereof. Lower substrate 102 is also provided with lower conductive layer 104 having optical transparency made from the material such as indium tin oxide, and formed in the same manner on a front surface thereof.
A plurality of dot spacers (not shown in the figures) is formed of an insulation resin at predetermined intervals on a front surface of lower conductive layer 104. There is a pair of upper electrodes 105 formed at two opposite sides of upper conductive layer 103. There is also a pair of lower electrodes 106 formed at two opposite sides of lower conductive layer 104 in a direction orthogonal to upper electrodes 105. Both upper electrodes 105 and lower electrodes 106 are made of an electrically conductive material such as silver.
In addition, spacer 107 of generally a frame-like shape is formed on either a lower peripheral surface of upper substrate 101 or an upper peripheral surface of lower substrate 102. Spacer 107 is provided with adhesive layers (not shown) coated on both a front surface and a back surface. Upper substrate 101 and lower substrate 102 are bonded at their peripheries with these adhesive layers, so that they face each other at a given space between them. Upper electrodes 105 and lower electrodes 106 are so formed that their terminal portions extend to one side of upper substrate 101 and lower substrate 102.
Wiring substrate 108 of a film-like form has a plurality of upper wiring patterns 109 (hereinafter referred to as patterns 109) formed on its front surface and a plurality of lower wiring patterns 110 (referred to as patterns 110) formed on its back surface. All of patterns 109 and 110 are made of an electrically conductive material such as silver, and constitute a plurality of wiring patterns. Both the front and the back surfaces of wiring substrate 108 is coated additionally with an insulation layer (not shown) to cover patterns 109 and 110. The insulating layer is not provided, however, on a terminal portion of wiring substrate 108, where connection is made to touch panel 100 and an electronic circuit (not shown) of an electronic apparatus. The terminal portion of wiring substrate 108 is placed and held in a position between upper substrate 101 and lower substrate 102.
Anisotropic conductive adhesive layer 111 (referred to as adhesive layer 111) contain a large number of electrically conductive particles 111b distributed in synthetic resin 111a. Adhesive layer 111 is used to bond and secures the terminal portion of wiring substrate 108 to upper substrate 101 and lower substrate 102. Upper electrodes 105 and terminal portion of patterns 109 are thus connected electrically through conductive particles 111b. Likewise, lower electrodes 106 and terminal portion of patterns 110 are connected electrically through conductive particles 111b. Touch panel 100 is thus constructed as described above.
Touch panel 100 constructed in this manner is placed in front of a liquid crystal display device or the like (not shown), and mounted to an electronic apparatus. In addition, the terminal portions of patterns 109 and 110 on wiring substrate 108 are connected to an electronic circuit of the electronic apparatus by such means as a connector and soldering.
In the structure discussed above, a front surface of upper substrate 101 is pressed to operate with a finger, a pen or the like tool while allowing visual observation of a screen on the liquid crystal display device provided on a back side of touch panel 100. This causes upper substrate 101 to deform, and upper conductive layer 103 to come in contact with lower conductive layer 104 at a position being pressed.
Voltages are applied successively from the electronic circuit to upper electrodes 105 through patterns 109 and to lower electrodes 106 through patterns 110 respectively. The electronic circuit detects the pressed position large number of conductive particles 111b link up across these electrodes. Such a phenomenon of short-circuiting, if occurs, may impair the electrical insulation between the electrodes or the wiring patterns, thereby resulting in instability of electrical connection and separation or operation of touch panel 100.
It is conceivable to reduce an amount of conductive particles 111b distributed in synthetic resin 111a in order to prevent short-circuiting between the electrodes or the wiring patterns attributable to development of aggregated portion 111C. When the added amount of conductive particles 111b is reduced, however, it becomes inevitable to lead instability in the electrical connections between upper electrodes 105 and patterns 109 as well as the electrical connections between lower electrodes 106 and patterns 110 through conductive particles 111b. 
It is for this reason to make adjustment of a size and amount of conductive particles 111b mixed in synthetic resin 111a, or conditions of the heating temperature and pressure used for thermal compression of adhesive layer 111. It may also become necessary that the anisotropic conductive adhesive is stirred thoroughly before it is applied to form adhesive layer 111, or adhesive layer 111 are inspected for presence of aggregated portion 111C. Any of the above makes the process of manufacturing touch panels 100 complicated.
Japanese Patent Unexamined Publication, No. 2003-58319, for instance, discloses conventional touch panel 100 of the kind discussed above.